This invention relates to the art of perforating sheet material. More particularly, it relates to the art of perfora-ting sheet material by directing a laser beam toward the sheet material to form holes therethrough. Even more particularly, it relates to the apparatus for forming multiple rows of such perforations along the length of the web of sheet material in a single pass. The apparatus of this invention is particularly suited for forming multiple rows of these holes or apertures through sheet material such as cigarette tipping paper.
In the prior art several techniques and types of apparatus have been used for perforating sheet material. Among these are the use of mechanical punches, electrical sparks, and projecting a laser beam through the apertures of a perforated drum over which the sheet material passes. In the tobacco industry most of the prior art techniques have, for various reasons, been found unsuitable or inefficient for perforating cigarette tipping paper. The perforations to be made in cigarette tipping paper are intended to control the delivery to the smoker of the cigarette smoke constituents by permitting air to be drawn in from the surrounding atmosphere to mix with the smoke during its passage through the filter structure. By controlling the mixing of the air and thus the delivery of the smoke constituents, various properties such as tar and nicotine levels in the cigarette can be regulated. Since the apertures are intended for the purpose of controlling such smoke constituent, it is important that the size and positioning of the holes be accurately controlled. Typically, these holes are on the order of 0.010 inches in diameter and spaced apart about 0.040 inches.
In attempting to achieve these desirable dimensions, the prior art structure has suffered numerous disadvantages. The most common type of prior art apparatus, mechanical pin punches, are extremely delicate in such a small size and frequently suffer damage to the punches themselves, as well causing tearing of the paper during perforating. The electric spark perforating apparatus burns holes in the paper, but may form irregular apertures and leave a charred edge around each aperture, both undesirable results.
One common type of prior art laser apparatus is exemplified in U.S. Pat. No. 3,808,394, in which a pulsed laser beam is directed toward a rotating mirror, which reflects the beam toward an opening in an independently rotating drum to vaporize the sheet material adjacent the opening. A more advanced form of laser apparatus is that disclosed in the co-pending Saunders et al. application, Ser. No. 670,113, which is assigned to the assignee of this application. The structure of Saunders et al. includes a rotating drum having apertures around its periphery, over which drum a narrow strip of the paper is passed. A continuous wave laser is directed against the interior of the drum periphery, projecting through the drum apertures to vaporize the sheet material adjacent thereto. The prior art laser apparatus has suffered from the disadvantage of low throughput and efficiency, since a single laser in that prior art is capable of forming only one row of apertures for each pass of the sheet material thereover. Accordingly, this prior art structure has generally been used, not with the 44 inch wide "parent" rolls of tipping paper, but with only a single bobbin of the paper having a width equivalent to that for only one, or at the most two, cigarette tips. In the rotating mask apparatus, the disadvantage has also flowed from the inefficient use of the continuous wave laser, in which only a portion of the output energy passes through the drum apertures to form holes, with the majority of the energy being reflected back to the laser or dissipated while performing no useful work. Obviously, the throughput of material could be increased dramatically if an entire parent roll, having paper of sufficient width for 32 cigarettes, could be perforated in a single pass through the perforating apparatus, and efficiency could be significantly improved if the entire energy of the laser beam could be utilized for perforating.